Many saline minerals are recognized as being commercially valuable. For example, trona, borates, potash and chlorides are mined commercially. After mining, these minerals need to be beneficiated to remove naturally occurring impurities.
With regard to trona (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O), high-purity trona is commonly used to make soda ash, which is used in the production of glass, paper and other goods. Naturally-occurring trona, or crude trona, is found in large deposits in the western United States, such as in Wyoming and California, and also in Egypt, Kenya, Botswana, Tibet, China, Venezuela and Turkey. The largest deposit of trona in the world is located in the Green River Basin in Wyoming. Crude trona ore from Wyoming is typically between about 80% and about 90% trona, with the remaining components including shortite, pyrite, quartz, dolomite, mudstone, oil shale, kerogen, mica, nahcolite and clay minerals.
Many areas of the glass and paper making industries require soda ash produced from trona having a purity of 99% or more. In order to obtain such a high purity, wet beneficiation processes have been used. Such processes generally involve crushing the crude trona, solubilizing the trona, treating the solution to remove insolubles and organic matter, crystallizing the trona, and drying the trona which may subsequently be calcined to produce soda ash. Alternatively, the crude trona can be calcined to yield crude sodium carbonate, which is then solubilized, treated to remove impurities, crystallized and dried to produce sodium carbonate monohydrate or calcined to produce soda ash. While the above-described wet processes can produce with high purity, they tend to be time consuming and expensive to perform, and therefore result in a product which is much more expensive than products produced using known dry processes.
Not all industries which use trona or soda ash require such a highly purified form. For example, certain grades of glass can be produced utilizing soda ash having less than 97% purity. For this purpose, U.S. Pat. No. 4,341,744 discloses a dry beneficiation process which is less complex and less expensive than the above-described wet beneficiation process. Such a dry beneficiation process generally includes crushing the crude trona, classifying the trona by particle size, electrostatically separating certain impurities, and optionally magnetically separating other impurities. Such a process can yield trona or soda ash having up to 95% to 97% purity with a 60-74% recovery, depending on the quantity and type of impurities present in the crude trona ore.
There are uses for trona or soda ash, for example in certain applications in the glass industry, requiring a purity of at least 97%, yet not needing a purity over 99%. To accomplish this, U.S. Pat. No. 5,470,554 discloses a dry method for beneficiating trona or soda ash that generally comprises the step of density separation, and optionally includes electrostatic separation and/or magnetic separation. U.S. Pat. No. 5,470,554 is incorporated herein by reference in its entirety. The resulting product can have trona or soda ash purities on the order of 97-98% or more. Although this process works quite satisfactorily, there is always a desire to reduce the number of steps, and thereby reduce the costs, associated with separation processes. In addition, there is always a desire to improve the recovery of beneficiation processes.
As such, it can be appreciated that it would be desirable to be able to beneficiate trona or soda ash utilizing a low cost dry beneficiation process to obtain trona or soda ash purities of about 97-98% or higher and recoveries on the order of 90-95%. Accordingly, it is an object of the present invention to provide a dry process for the beneficiation of trona or soda ash resulting in higher purities and lower costs than many existing dry beneficiation processes, and which is simpler and less expensive than known wet beneficiation processes.